Well, I have to admit that I've been wrong. I was hoping that someone here could give an explanation, rather than just telling me the way that it is, but I understand if you don't have the time.

Let me explain it to those that I've possibly led astray.

A surface between two mediums is a radiation filter. What this means is that it will reflect radiation or transmit radiation through the surface, depending on its properties (see Geoff's list). Some materials are better than others, etc. What is important to understand though (and what I was having trouble with) is the fact that the filter is the same for both directions. For example, a shiny object that reflects radiation away from it, also reflects internal radiation back into it. Let me try a picture:

Heat coming in from the other direction will transmit and reflect to exactly the same proportions.

This is important, because otherwise the laws of thermodynamics will not be conserved. Otherwise, one could imagine a surface that had perfect transmittance one direction and perfect reflection the other. This fictional paint would cause objects to begin heading towards absolute zero, or become infinitely hot. Obviously this is not the case.

If you are having trouble with a heat sink that is a little too small, you may try painting it black before giving up. It may make the difference.

As far as chroming: I have to admit that it would look cool. Even though it would be a litttle warmer than a black one.